If something doesn’t work (for example a compiler flag, a libc function, etc.) then first search the comprehensive documentation on this site.

Next check if there is a test for the failing functionality in the Emscripten test suite (run grep-r in tests/). All the tests are known to pass on the master branch, so they provide concrete “known-good” examples of how various options and code are used.

Emscripten makes some trade-offs that make the generated code faster and smaller, at the cost of longer compilation times. For example, we build parts of the standard library along with your code, which enables some additional optimizations, but takes a little longer to compile.

Note

You can determine what compilation steps take longest by compiling with EMCC_DEBUG=1 in the environment and then reviewing the debug logs (by default in /tmp/emscripten_temp). Note that compiling in debug mode takes longer than normal, because we print out a lot of intermediate steps to disk.

Emscripten can run some passes in parallel (specifically, the JavaScript optimisations). Increasing the number of cores results in an almost linear improvement.

Emscripten will automatically use more cores if they are available. You can control how many cores are used with EMCC_CORES=N (this is useful if you have many cores but relatively less memory).

Make sure that the native optimizer is being used, which greatly speeds up optimized builds as of 1.28.2. EMCC_DEBUG=1 output should not report errors about the native optimizer failing to build or not being used because of a previous failed build (if it previously failed, do emcc--clear-cache then compile your file again, and the optimizer will be automatically rebuilt).

When you have multiple bitcode files as inputs, put the largest file first (LLVM linking links the second and later ones into the first, so less copying is done on the first input to the linker).

Having fewer bitcode files can be faster, so you might want to link files into larger files in parallel in your build system (you might already do this if you have logical libraries), and then the final command has fewer things to operate on.

You don’t need to link into a single bitcode file yourself, you can call the final emcc command that emits JS with a list of files. emcc can then defer linking and avoid an intermediary step, if possible (this optimization is disabled by LTO and by EMCC_DEBUG=2).

Make sure you build with -O2 so code is optimized and minified. You should also set up gzip compression on your webserver, which all browsers now support.

Note

You can use the closure compiler to reduce code size even further (--closure1). However that will require that your code be prepared for closure compiler advanced optimizations, including proper exports and so forth. It is usually not worth the effort over an optimized build and supporting gzip on your webserver.

Why does compiling code that works on another machine gives me errors?¶

Make sure you are using the Emscripten bundled system headers. Using emcc will do so by default, but problems may occur if you use your local system headers with emcc or compile into LLVM bitcode yourself.

Network latency is also a possible factor in startup time. Consider putting the file loading code in a separate script element from the generated code so that the browser can start the network download in parallel to starting up the codebase (run the file packager and put file loading code in one script element, and the generated codebase in a later script element).

Why does my code fail to compile with an error message about inline assembly (or {"text":"asm"})?¶

Emscripten cannot compile inline assembly code (because it is CPU specific, and Emscripten is not a CPU emulator).

You will need to find where inline assembly is used, and disable it or replace it with platform-independent code.

Note

Emscripten automatically unsets the following #define values, as these are commonly set in projects to enable platform dependent code (inline assembly):

The browser event model uses co-operative multitasking — each event has a “turn” to run, and must then return control to the browser event loop so that other events can be processed. A common cause of HTML pages hanging is JavaScript that does not complete and return control to the browser.

Graphical C++ apps typically have an infinite main loop in which event handling, processing and rendering is done, followed by a delay to keep the frame-rate right (SDL_DELAY in SDL apps). As the main loop does not complete (is infinite) it cannot return control to the browser, and the app will hang.

Apps that use an infinite main loop should be re-coded to put the actions for a single iteration of the loop into a single “finite” function. In the native build this function can be run in an infinite loop as before. In the Emscripten build it is set as the main loop function and will be called by the browser at a specified frequency.

System libraries that are included with Emscripten are automatically linked when you compile (just the necessary parts). This includes libc, libc++ (C++ standard library) and SDL.

Libraries not included with Emscripten (like Boost) must be compiled and linked with the program just as if they were a module in the project.

There is a set of libraries ported to Emscripten for convenient use, Emscripten Ports. See Building Projects

Another option is to implement needed C APIs as JavaScript libraries (see --js-library in emcc and Implement a C API in JavaScript). Emscripten itself does this for libc (not including malloc) and SDL (but not libc++ or malloc).

Note

Unlike other compilers, you don’t need -lSDL to include SDL (specifying it will do no harm).

In the specific case of Boost, if you only need the boost headers then you don’t need to compile anything.

Emscripten-generated code running in the browser cannot access files in the local file system. Instead you can use preloading and embedding to work around the lack of synchronous file IO. See File System Overview for more information.

It is possible to allow access to local file system for code running in node.js, use the NODEFS filesystem option.

How can I tell when the page is fully loaded and it is safe to call compiled functions?¶

(You may need this answer if you see an error saying something like youneedtowaitfortheruntimetobeready(e.g.waitformain()tobecalled), which is a check enabled in ASSERTIONS builds.)

Calling a compiled function before a page has fully loaded can result in an error, if the function relies on files that may not be present (for example the .mem file and preloaded files are loaded asynchronously, and therefore if you just place some JS that calls compiled code in a --post-js, that code will be called synchronously at the end of the combined JS file, potentially before the asynchronous event happens, which is bad).

The easiest way to find out when loading is complete is to add a main() function, and within it call a JavaScript function to notify your code that loading is complete.

Note

The main() function is called after startup is complete as a signal that it is safe to call any compiled method.

For example, if allReady() is a JavaScript function you want called when everything is ready, you can do:

#include<emscripten.h>intmain(){EM_ASM(allReady());}

Another option is to define an onRuntimeInitialized function,

Module['onRuntimeInitialized'] = function() { ... };

That method will be called when the runtime is ready and it is ok for you to call compiled code. In practice, that is exactly the same time at which main() would be called, so onRuntimeInitialized doesn’t let you do anything new, but you can set it from JavaScript at runtime in a flexible way.

The crucial thing is that Module exists, and has the property onRuntimeInitialized, before the script containing emscripten output (my_project.js in this example) is loaded.

Another option is to use the MODULARIZE option, using -sMODULARIZE=1. That will put all of the generated JavaScript in a function, which you can call to create an instance. The instance has a promise-like .then() method, so if you build with say -sMODULARIZE=1-s'EXPORT_NAME="MyCode"' (see details in settings.js), then you can do something like this:

MyCode().then(function(Module){// this is reached when everything is ready, and you can call methods on Module});

(You may need this answer if you see an error saying something like atexit()called,butNO_EXIT_RUNTIME or stdiostreamshadcontentinthemthatwasnotflushed.youshouldsetNO_EXIT_RUNTIMEto0.)

By default Emscripten sets NO_EXIT_RUNTIME=1, which means that we don’t include code to shut down the runtime. That means that when main() exits, we don’t flush the stdio streams, or call the destructors of global C++ objects, or call atexit callbacks. This lets us emit smaller code by default, and is normally what you want on the web: even though main() exited, you may have something asynchronous happening later that you want to execute.

In some cases, though, you may want a more “commandline” experience, where we do shut down the runtime when main() exits. You can build with -sNO_EXIT_RUNTIME=0, and then we will call atexits and so forth. When you build with ASSERTIONS, you should get a warning when you need this. For example, if your program prints something without a newline,

#include<stdio.h>intmain(){printf("hello");// note no newline}

If we don’t shut down the runtime and flush the stdio streams, “hello” won’t be printed. In an ASSERTIONS build you’ll get a notification saying stdiostreamshadcontentinthemthatwasnotflushed.youshouldsetNO_EXIT_RUNTIMEto0.

Why do functions in my C/C++ source code vanish when I compile to JavaScript, and/or I get Nofunctionstoprocess?¶

Emscripten does dead code elimination of functions that are not called from the compiled code. While this does minimize code size, it can remove functions that you plan to call yourself (outside of the compiled code).

To make sure a C function remains available to be called from normal JavaScript, it must be added to the EXPORTED_FUNCTIONS using the emcc command line. For example, to prevent functions my_func() and main() from being removed/renamed, run emcc with:

./emcc-s"EXPORTED_FUNCTIONS=['_main', '_my_func']"...

Note

_main should be in the export list, as in that example, if you have a main() function. Otherwise, it will be removed as dead code; there is no special logic to keep main() alive by default.

Note

EXPORTED_FUNCTIONS affects compilation to JavaScript. If you first compile to an object file,
then compile the object to JavaScript, you need that option on the second command.

If your function is used in other functions, LLVM may inline it and it will not appear as a unique function in the JavaScript. Prevent inlining by defining the function with EMSCRIPTEN_KEEPALIVE:

voidEMSCRIPTEN_KEEPALIVEyourCfunc(){..}

EMSCRIPTEN_KEEPALIVE also exports the function, as if it were on EXPORTED_FUNCTIONS.

Note

All functions not kept alive through EXPORTED_FUNCTIONS or EMSCRIPTEN_KEEPALIVE will potentially be removed. Make sure you keep the things you need alive using one or both of those methods.

Decorating your code with EMSCRIPTEN_KEEPALIVE can be useful if you don’t want to have to keep track of functions to export explicitly, and when these exports do not change. It is not necessarily suitable for exporting functions from other libraries — for example it is not a good idea to decorate and recompile the source code of the C standard library. If you build the same source in multiple ways and change what is exported, then managing exports on the command line is easier.

Running emcc with -sLINKABLE=1 will also disable link-time optimizations and dead code elimination. This is not recommended as it makes the code larger and less optimized.

Another possible cause of missing code is improper linking of .a files. The .a files link only the internal object files needed by previous files on the command line, so the order of files matters, and this can be surprising. If you are linking .a files, make sure they are at the end of the list of files, and in the right order amongst themselves. Alternatively, just use .so files instead in your project.

Tip

It can be useful to compile with EMCC_DEBUG=1 set for the environment (EMCC_DEBUG=1emcc... on Linux, setEMMCC_DEBUG=1 on Windows). This splits up the compilation steps and saves them in /tmp/emscripten_temp. You can then see at what stage the code vanishes (you will need to do llvm-dis on the bitcode stages to read them, or llvm-nm, etc.).

Why is the File System API is not available when I build with closure?¶

The Closure Compiler will minify the File Server API code. Code that uses the file system must be optimized with the File System API, using emcc’s --pre-jsoption.

Why does my code break and gives odd errors when using -O2--closure1?¶

The Closure Compiler minifies variable names, which results in very short variable names like i, j, xa, etc. If other code declares variables with the same names in global scope, this can cause serious problems.

This is likely to be the cause if you can successfully run code compiled with -O2 set and --closure unset.

One solution is to stop using small variable names in the global scope (often this is a mistake — forgetting to use var when assigning to a variable).

Another alternative is to wrap the generated code (or your other code) in a closure, as shown:

The likely cause is an undefined function — a function that was referred to, but not implemented or linked in. If you get undefined, look at the line number to see the function name.

Emscripten by default does not give fatal errors on undefined symbols, so you can get runtime errors like these (because in practice, for many codebases it is easiest to get them working without refactoring them to remove all undefined symbol calls). If you prefer compile-time notifications, run emcc with -sWARN_ON_UNDEFINED_SYMBOLS=1 or -sERROR_ON_UNDEFINED_SYMBOLS=1.

Aside from just forgetting to link in a necessary object file, one possible cause for this error is inline functions in headers. If you have a header with inlineintmy_func(){..} then Clang may not actually inline the function (since inline is just a hint), and also not generate code for it (since it’s in a header). The result is that the generated bitcode and JavaScript will not have that function implemented. One solution is to add static to the function declaration, which forces code to be generated in the object file: staticinlineintmy_func(){..}.

The Module object will contain exported methods. For something to appear there, you should add it to EXPORTED_FUNCTIONS for compiled code, or EXTRA_EXPORTED_RUNTIME_METHODS for a runtime method (like getValue). For example,

./emcc-s"EXPORTED_FUNCTIONS=['_main', '_my_func']"...

would export a C method my_func (in addition to main, in this example). And

./emcc-s"EXTRA_EXPORTED_RUNTIME_METHODS=['ccall']"...

will export ccall. In both cases you can then access the exported function on the Module object.

Note

You can use runtime methods directly, without exporting them, if the compiler can see them used. For example, you can use getValue in EM_ASM code, or a --pre-js, by calling it directly. The optimizer will not remove that JS runtime method because it sees it is used. You only need to use Module.getValue if you want to call that method from outside the JS code the compiler can see, and then you need to export it.

Note

Emscripten used to export many runtime methods by default. This increased code size, and for that reason we’ve changed that default. If you depend on something that used to be exported, you should see a warning pointing you to the solution, in an unoptimized build, or a build with ASSERTIONS enabled, which we hope will minimize any annoyance. See Changelog.markdown for details.

Why does Runtime no longer exist? Why do I get an error trying to access Runtime.someThing?¶

1.37.27 includes a refactoring to remove the Runtime object. This makes the generated code more efficient and compact, but requires minor changes if you used Runtime.* APIs. You just need to remove the Runtime. prefix, as those functions are now simple functions in the top scope (an error message in -O0 or builds with assertions enabled with suggest this). In other words, replace

x=Runtime.stackAlloc(10);

with

x=stackAlloc(10);

Note

The above will work for code in a --pre-js or JS library, that is, code that is compiled together with the emscripten output. If you try to access Runtime.* methods from outside the compiled code, then you must export that function (using EXTRA_EXPORTED_RUNTIME_METHODS), and use it on the Module object, see that FAQ entry.

Why do I get a NameError or aproblemoccurredinevaluatingcontentaftera"-s" when I use a -s option?¶

# this works in the shell on most Linuxes and on macOS
./emcc a.c -s "EXTRA_EXPORTED_RUNTIME_METHODS=['addOnPostRun']"
# or you may need something like this in a Makefile
./emcc a.c -s EXTRA_EXPORTED_RUNTIME_METHODS=\"['addOnPostRun']\"

The proper syntax depends on the OS and shell you are in, and if you are writing in a Makefile, etc.

Why do I get an odd python error complaining about libcxx.bc or libcxxabi.bc?¶

A possible cause is that building libcxx or libcxxabi failed. Go to system/lib/libcxx (or libcxxabi) and do emmakemake to see the actual error. Or, clean the Emscripten cache (./emcc--clear-cache) and then compile your file with EMCC_DEBUG=1 in the environment. libcxx will then be built in /tmp/emscripten_temp/libcxx, and you can see configure*,make* files that are the output of configure and make, etc.

Another possible cause of this error is the lack of make, which is necessary to build these libraries. If you are on Windows, you need cmake.

Why does running LLVM bitcode generated by emcc through lli break with errors about impure_ptr?¶

Note

lli is not maintained, and has odd errors and crashes. We do include tools/nativize_llvm.py (which compiles bitcode to a native executable) but it will also hit the impure_ptr error.

The issue is that newlib uses impure_ptr code, while glibc uses something else. The result is that bitcode built with the Emscripten will not run locally unless your machine uses newlib (basically, only embedded systems).

The impure_ptr error only occurs during explicit use of stdout etc., so printf(..) will work, but fprintf(stdout,..) will not. Usually it is simple to modify your code to avoid this problem.

Why do I get a stack size error when optimizing: RangeError:Maximumcallstacksizeexceeded or similar?¶

Why do I get error:cannotcompilethisaggregateva_argexpressionyet and it says compilerfrontendfailedtogenerateLLVMbitcode,halting afterwards?¶

Building Fastcomp from source (and hence the SDK) can fail at 100% progress. This is due to out of memory in the linking stage, and is reported as an error: collect2:error:ldterminatedwithsignal9[Killed].

The solution is to ensure the system has sufficient memory. On Ubuntu 14.04.1 LTS 64bit, you should use at least 6Gb.

By default Emscripten uses doubles for all floating-point variables, that is, 64-bit floats even when C/C++ code contains 32-bit floats. This is simplest and most efficient to implement in JS as doubles are the only native numeric type. As a result, you may see rounding errors compared to native code using 32-bit floats, just because of the difference in precision between 32-bit and 64-bit floating-point values.

To check if this is the issue you are seeing, build with -sPRECISE_F32=1. This uses proper 32-bit floating-point values, at the cost of some extra code size overhead. This may be faster in some browsers, if they optimize Math.fround, but can be slower in others. See src/settings.js for more details on this option.

Emscripten output by default is just some code. When put in a script tag, that means the code is in the global scope. So multiple such modules on the same page can’t work.

But by putting each module in a function scope, that problem is avoided. Emscripten even has a compile flag for this, MODULARIZE, useful in conjunction with EXPORT_NAME (details in settings.js).

However, there are still some issues if the same Module object (that defines the canvas, text output area, etc.) is used among separate modules. By default Emscripten output even looks for Module in the global scope, but when using MODULARIZE, you get a function you must call with the Module as a param, so that problem is avoided. But note that each module will probably want its own canvas, text output area, etc.; just passing in the same Module object (e.g. from the default HTML shell) may not work.

So by using MODULARIZE and creating a proper Module object for each module, and passing those in, multiple modules can work fine.

Another option is to use an iframe, in which case the default HTML shell will just work, as each will have its own canvas, etc. But this is overkill for small programs, which can run modularly as described above.